A non-aqueous secondary battery such as a lithium-ion secondary battery is known as having a high voltage and a high capacity. From these reasons, it is widely adopted as a power source of various portable equipments. In addition, recent years have been spreading into medium-scale or large-scale size applications, including power tools such as electric power tools, and electric vehicles and electric bicycles.
Especially, it is used in cellular phones or game machines that have advanced its development in miniaturization and multifunction. The batteries in such applications are further demanded to increase their capacity. One approach is in the research and development to focus on the electrode active materials exhibiting a high charge discharge capacity. Among other things, a focus has been made on a material that allows more absorption and desorption of lithium (ions), such as silicon (Si) and tin (Sn) as a negative electrode active material, instead of a carbonaceous material such as graphite which is adopted in conventional non-aqueous secondary batteries. In particular, SiOx was reported to have showed excellent load characteristics in a structure of ultrafine Si particles dispersed in the SiO2. See Patent Publications Nos. 1 and 2.
However, SiOx exhibits large volumetric expansion and shrinkage associated with the charge and the discharge reaction. Therefore, the particles can be crushed during each cycle of the charge and the discharge of the battery. As a result, it is known that exposed surfaces of Si can react with the solvent of the non-aqueous electrolyte, causing the problems that increase an irreversible capacity or brings about the battery swelling due to the charge and the discharge. Also, SiOx has a fine shape, so that it can attain a certain level of improvements in the load characteristic of the battery. It is, however, noted that SiOx per se is a material having a low conductivity. Therefore, there is still a room to make further improvements.
Under the circumstances as explained above, it was proposed to restrict the use rate of SiOx to suppress the volumetric expansion and shrinkage associated with the charge and the discharge reactions. It was also proposed to coat the SiOx surfaces with a conductive material such as carbon to improve the load characteristics. It was also proposed to use a non-aqueous electrolyte including a halogen-substituted cyclic carbonate (e.g., 4-fluoro-1,3-dioxolane-2-one). Using these technical proposals, the batteries have been proposed to improve the charge discharge cycle characteristics thereof. See Patent Document No. 3.